Hot Rod Farmer: Should a listener convert a TBI engine to TPI?September 14, 2022
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This week a question is posed wondering if an engine should be switched from a wet flow electronic fuel injection system to a dry flow design. In typical Hot Rod Farmer fashion, Ray Bohacz, provides his best advice and explains his thoughts technically, though it may not be the answer the person was looking for.
Hey Ray, Thanks for the answer on your podcast. For anyone unfamiliar with the subject you did a fantastic job of explaining the capabilities and limitations of Tuned Port Injection and the importance of formulating a goal and developing a plan. From the factory the 305 TBI made 170HP/255TQ; the 305 TPI made 205/285. With my free flowing exhaust, cold air intake and modern ECM to dial in the tune, I was hoping to gain every bit of that 35 HP and 30 ft/lbs and a little more. My goal is simply to increase the capability to move in traffic, for as low a cost as possible, and still have a fun & interesting project.
I tried to keep my inquiry short, without a lot of background to get your thoughts on using the TBI swirl-port heads in conjunction with the TPI. Those heads have a “ramp” cast into the intake port near the valve that induces swirl as the air fuel mixture is drawn into the cylinder. This ramp is an RPM limiting factor, for different reasons than the long runners of the TPI. I realize that a 5 angle valve job is key to increasing low lift flow, which is key for short duration cams to develop low RPM torque. (I am considering other cams as well). But what I wanted to know is, if using these heads would enhance/magnify the low end torque characteristic of the engine, or be an impediment to the long runner design of the TPI.
All of your info on heat, cubic inches, gears, carbs etc. was spot on. A carbureted 350 or 383 with 3:42 gears and 27” tall tires would get me the drivability I desire the easy way, but money is always a factor. I recently screwed together a “low-buck” aluminum headed, carb’d 350 for my son’s S10 that ended up costing about $4,500. I was hoping to spend less than half that to maximize the torque capability of the 305. A used TPI goes for about $400 around the country (in varying conditions). The cost to refurbish could be as much as $500 if I have to fix every bushing, bolt hole, sensor etc. The stand-alone controller is $800. I have built a couple cold/ram air induction systems on other cars and would NEVER consider feeding a TPI with an under hood cone filter. I project the project total to be around $1700.
I have yet to find someone I trust to put gears in or I would have already done that. Gears would be less than $1700… but not by much. (Setting up a rear end is about the only thing I can’t do myself). Anyway, thank you for taking the time to work through all the issues. I am definitely reconsidering the cam. The B-Body LT1 cam has 191/196 duration with 418/430 lift on a 111 LSA. It is almost identical to the L31 cam. That LSA is the downside of that cam; TPI cams were 114-117 LSA depending on year model. By the way the TBI cam has 178/194 duration, 350/385 lift (Yes, a 350!! Intake Lift!!) on a 109 LSA. The up side to the LT1 and L31 cams are that I have both in my garage. If I buy a better cam it will add around $300-$400 and still make the project half the price of building a carb’d small block. Thanks again Ray. You’re a great American and a brother in Christ.